There's a moment during CSNY: Déjà Vu, Neil Young's document of the supergroup's 2006 Freedom Of Speech tour, when one furious ticket holder outside the Philips Arena, Atlanta spits: "Crosby, Stills, Nash and Young can suck my fucking dick!"

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Atlanta is a Republican heartland, a “red state”, and many at the show seem unwilling to accept CSNY as politically engaged firebrands, particularly when the lyrics to “Impeach The President” – “Let's impeach the President for lying and misleading our country into war” – are displayed on giant video screens behind the band.

As the camera crew attract similarly vitriolic comments from other disgruntled punters – one report suggests almost a third of the audience have walked out – I'm left wondering quite what these people expected from CSNY. They are, after all, a band with a lengthy track record of political activism, caught up in the anti-Vietnam movement (as we’re reminded in archive footage dotted throughout the film), while the Freedom Of Speech tour is coming off the back of Young’s Living With War album, arguably the most overtly political record in his career.

But perhaps, to a lot of those people who stormed out of the Philips Arena, CSNY are now no different from a host of other bands on the enormodome circuit. All Greatest Hits packages, an easy, nostalgic stroll down memory lane, folks expecting those intricate harmonies and some choice FM radio cuts to sing along to, the memories of “Ohio” and “Find The Cost Of Freedom” presumably strategically airbrushed from memory.

Anyway, this section – as the band circle round the American south – seems to me to be the most intriguing part of the film. It’s interesting just how far from their counterculture roots people seem to think CSNY have strayed over the years, as if age, expanding waistlines, receding hairlines and multi-millionaire status somehow precludes them from having political opinions anymore. Earlier in the film, we see CSNY play places like New York and other left-leaning, Democratic strongholds where their sentiments are widely supported; but down south, they’re on something approximating a front line, raising a shitstorm of controversy.

Apparently, during those dates in the south, there were bomb-sniffing dogs at the shows and guards outside Young’s hotel room (though footage of this, if it was ever filmed, never makes it into the film). “It was the most hair-raising, nerve-wracking, terrible experience,” he’s said.

In fact, driven by Young, the whole band seems galvanised by their latest mission, even if Stephen Stills does express doubts about the enterprise early on. We see home movie footage of Young in what looks like his living room, writing and recording the whole Living With War album in nine days. Snippets from chat shows, with a friendly, laid-back Young chatting amiably about bringing down Bush and his opposition to the war in Iraq. We hear from Crosby and Nash, all very on-message with Young; Stills, also, before too long. If I have a fault with the film, I’d like to have seen more of the offstage dynamic between the band. There’s some interesting footage of Young, his arm round Stills’ shoulders, gently cajoling him, almost like an elder brother affectionately ribbing a younger sibling. When you consider the often-fractious history the two men have had, you want to know a little more about where their relationship is currently at, and by extension the rapport between the four of them when off duty.

At this point, the film pretty much changes course. CSNY had taken on the road with them a former Vietnam veteran turned ABC news correspondent Michael Cerre, who’d covered the war in Afghanistan and Iraq and was now embedded with the band, out filming the audience responses during the shows and the accompanying backstage footage. He also catches up with some of the servicemen and women he’d met in the field, all of them now firmly opposed to the war, some of them disabled, for what feels like the human interest strand in a current affairs’ programme. For instance, we meet Vets For Vets – not, sadly, the animal medical profession offering succour to the good servicemen and women of the US military, but a support group run by Vietnam veterans for soldiers returning from Afghanistan and Iraq as they readjust to life after the horrors of war. It’s occasionally pretty moving viewing, Cerre never letting the material slip into sanctimonious tub-thumping.

Initially, there’s something fairly schizophrenic, then, about CSNY: Déjà Vu, as it morphs from concert film to human interest documentary. But by dwelling on the lives of the veterans affected by the conflict, Young and Cerre make explicit the connection between the material and its roots in the War on Terror.

CSNY: Déjà Vu opens in the UK this Spring